1.Definition: appealing to the mind

"A serious book"

Related Adjective(s):good

2.Definition: causing fear or anxiety by threatening great harm

"A serious wound", "A serious turn of events"

Related Adjective(s):dangerous, grave, grievous, severe

3.Definition: completely lacking in playfulness

Related Adjective(s):sober, unplayful

4.Definition: concerned with work or important matters rather than play or trivialities

"A serious student of history", "A serious attempt to learn to ski", "Gave me a serious look", "A serious young man", "Are you serious or joking?", "Don't be so serious!"

5.Definition: of great consequence

"Marriage is a serious matter"

6.Definition: requiring effort or concentration; complex and not easy to answer or solve

"Raised serious objections to the proposal", "The plan has a serious flaw"